One of the more prevalent types of freezers used to provide cryogenic freezing of a product (e.g. foodstuffs) is a continuous, in-line tunnel that utilizes liquid nitrogen as an expendable refrigerant. One such apparatus in commercial use is shown in U.S. Pat. Nos. 3,813,895 and 3,892,104, the specifications of both patents being incorporated herein by reference. The apparatus of the prior art can achieve high thermal efficiency because it is designed as a counterflow heat exchanger. The product moves through the tunnel on a continuous belt from an entry end (portal or opening) to a discharge end (portal or opening). Liquid nitrogen is sprayed onto the food product at a location adjacent to the discharge end (opening) of the freezer. The cold nitrogen gas, at -320.degree. F. (-196.degree. C.), evolved in the liquid nitrogen spray zone, moves through multiple zones of gas recirculation as it flows toward the entrance of the freezer. Since the maximum available refrigeration has been utilized at that point, the warmed nitrogen gas can then be vented to the outside atmosphere by an exhaust fan placed proximate the entry end of the tunnel.
Liquid nitrogen that is in equilibrium at 35.0 psia (241 kpa) has a latent heat of 80.5 BTU/lb. (187 J/g) when vaporized at atmospheric pressure. When the product enters the freezer at 75.degree. F. (24.degree. C.), the nitrogen gas will leave the freezer entrance at approximately 0.degree. F. (-18.degree. C.) in a freezer such as shown in the aforementioned patents and offered for sale by Air Products and Chemicals, Inc. as a CRYO-QUICK freezer. At these conditions the freezer is operating at optimum thermal efficiency and the nitrogen gas will have a sensible heat of 79.5 BTU/lb. (185 J/g). Thus, the liquid nitrogen has a total available refrigeration of 160 BTU/lb. (372 J/g). Since the sensible heat of the nitrogen gas is almost one-half of the total available refrigeration, it is necessary to provide correct nitrogen gas flow through the freezer to achieve high thermal efficiency.
The amount of liquid nitrogen injected into the freezer will depend upon the amount of refrigeration required by the product to be frozen (e.g. foodstuff). Further, whenever production is interrupted, the liquid nitrogen flow rate should be reduced substantially to maintain the freezer at its operating temperature. In a typical CRYO-QUICK freezer, having a conveyor belt 28" (711 mm) width and a length of 66' (20 m), the liquid nitrogen flow rate will vary from 3065 to 358 lb/hr (1390 to 162 kg/hr). In addition, the most efficient operation is obtained when the liquid nitrogen flow is shut off completely during the production interruption. If the production is stopped for a long period of time, then liquid nitrogen is readmitted to the freezer based upon the temperature within the freezer. Thus, the nitrogen gas flow through the freezer must change over a wide range from the maximum flow to zero flow.
If the gas flow control system moves a larger volume of gas than the amount of gaseous nitrogen evolved in the liquid nitrogen spray zone, warm room air will be pulled into the discharge opening of the freezer. The entry of warm room air will be a significant heat input, causing a loss of thermal efficiency. Further, the moisture contained in the room air will result in frost and ice accumulation within the freezer and impair its performance. If the gas flow control system moves a smaller volume than required, cold nitrogen gas will spill out of the discharge opening, causing a significant loss in thermal efficiency. Also, the nitrogen gas spilling into the processing room can cause an oxygen deficient condition that could result in a serious safety hazard.
In early freezers represented by U.S. Pat. No. 3,345,828, to insure that the cold gas would flow countercurrent to the product flow, parallel fans were employed in the tunnel. A thermocouple placed at the collection point of cold gas, where it interfaces with warm gas, was used to detect the level of the hot/cold interface and to change position of a damper (76) to equalize volume of circulation between the parallel flow fans. While this method proved satisfactory for freezers employing parallel flow fans, patentees in U.S. Pat. No. 3,403,527 improved this apparatus by employing additional dampers with the parallel flow fans.
Subsequent to the early parallel flow fan type freezers, it was discovered that a radial flow fan could be used to force the gas in countercurrent flow to the product. U.S. Pat. No. 3,813,895 discloses the type of freezer using all radial fans wherein a curved damper, which is temperature actuated, can be used to control the total flow of gas in the freezer. However, it was found that this apparatus performed satisfactorily on freezers of small dimensions (e.g. tunnel length of 22 ft. or less). The patentees in U.S. Pat. No. 3,892,104 employed a centrifugal fan to move the cold cryogen toward the entry end of the tunnel. Control of the fan and hence control of the movement of gas through the tunnel was effected by sensing the spray header pressure which in turn controlled the speed of the fan.
U.S. Pat. No. 4,528,819 discloses an immersion-type cryogenic freezer suitable for freezing foodstuffs wherein movement of the vaporized cryogen is in concurrent flow with the movement of the product through the freezer. Patentees disclose control of an exhaust fan to control the direction of vaporized nitrogen flow, which in turn prevents air insufflation into the freezer. However, an exhaust fan cannot be used effectively in a tunnel type freezer to move the vaporized cryogen through the freezer. When the freezer is more than 30 ft long, the exhaust fan is unable to move a sufficient volume of vaporized cryogen through the freezer. Although an exhaust fan could be used on smaller freezers, the exhaust fan will also pull room air through the entry end opening of the freezer. When moist room air is mixed with the vaporized cryogen, the moisture will become frost that will clog the exhaust duct. This condition is most severe when the vaporized cryogen is colder than -50.degree. F. and the relative humidity of the room air is greater than 50%.
U.S. Pat. No. 3,613,386 discloses and claims a control system for regulating liquid nitrogen flow in a cryogenic freezer. The control system disclosed in the U.S. Pat. No. 3,613,386 patent is used in the radial-type freezers sold today and can be utilized with the control system of the present invention.
The commercial CRYO-QUICK freezer employs a gas flow control system such as described in U.S. Pat. No. 3,892,104. A freezer of this type with variable speed gas control system directs the flow of vaporized nitrogen by sensing the pressure in the liquid nitrogen spray header. The pressure signal is then used to change the speed of the gas control blower, which in this case is a centrifugal fan. This system, although it will operate correctly during continuous production, has several disadvantages. When the food product first enters the freezer, the pressure drop through the freezer changes until the conveyor belt is completely covered through its entire path inside the freezer. As a result, the freezer operator must adjust the maximum speed potentiometer each time production is started. In the same manner, the flow condition throughout the freezer changes whenever production is stopped. Thus, the freezer operator must again adjust the maximum speed potentiometer as the freezer is emptied of product. Experienced users of this type of equipment have found the pressure drop through the freezer changes for different food products. Thus, when different food products are loaded into the freezer, the freezer operator must readjust the maximum speed potentiometer to achieve correct nitrogen gas flow through the freezer. If the equilibrium conditions of the liquid nitrogen, as indicated by the liquid nitrogen storage tank pressure, change significantly, the quality of the liquid nitrogen flowing through the spray nozzles will also change. It is for this reason that the liquid nitrogen spray header pressure will be different for the same liquid nitrogen flow rate. This same condition will obtain if the liquid nitrogen spray nozzles become clogged with debris. Under these circumstances the freezer operator must then readjust the maximum speed potentiometer to achieve correct gas flow. The most serious disadvantage of the present system is that it requires the freezer operator to adjust the maximum speed potentiometer for proper operation. If the freezer operator adjusts the system incorrectly, the freezer will operate inefficienty until the system is readjusted.